Emissions regulations in the United States have resulted in changes to coal-based electric generating plants through the addition of emission controls.
During combustion of coal in coal-fired systems, combustion products/byproducts are generated and entrained in exhaust gases, sometimes referred to flue gases. These combustion byproducts include fly ash comprising lightweight particulate matter; and gaseous compounds such as sulfur dioxide (SO2), sulfur trioxide (SO3), hydrochloric acid (HCl), and hydrofluoric acid (HF). These gaseous combustion byproducts may become air pollutants if emitted to the atmosphere. Control of SO2/SO3 emissions (commonly referred to as ‘SOx’ emissions) and HCl/HF emissions requires removal of these gaseous compounds from flue gases prior to release of the flue gases into the environment. Many materials have been employed to treat the flue gases. The physical nature of these materials varies from wet scrubbing to injection of dry powdered materials and is dependent upon the overall pollution control process system employed.
The gaseous combustion byproducts are generally acidic, and thus slurries or dry materials used to remove (“scrub”) them from the flue gases are alkaline. Wet removal systems (referred to as ‘scrubbers’) used for flue gas desulfurization typically utilize aqueous slurries of lime-based reagents (e.g., calcium oxide) or limestone to neutralize the sulfurous and/or sulfuric acids produced from the dissolution and subsequent oxidation of flue gas in scrubbers. The reaction taking place in wet scrubbing of SO2 using a CaCO3 (limestone) slurry or a lime-based slurry (Ca(OH)2) produces CaSO3 (calcium sulfite).
When using wet scrubbers employing limestone slurries or lime-based reagents, large volumes of waste product are produced and must be hauled away for disposal. Such practice is common among power plants located in areas where landfill space is abundant or is a cost-effective disposal alternative.
With SO2 control for NAAQS, CSAPR, Regional Haze, consent orders, and permits becoming more widespread at facilities not well suited for wet scrubbing, dry sorbent injection (DSI) technology with sodium sorbents represents a cost effective solution. Ash leachate issues are a concern and while many users have no difficulty landfilling ash with high sodium content, there are some situations where ash treatment could be beneficial.
Recently, other alkali materials have gain acceptance in lieu of or in addition of lime-based reagents and limestone which offer flexibility and versatility in the operation of emission controls, maintenance and waste disposal requirements of flue gas desulfurization scrubber systems. These other materials are typically more expensive, but also more efficient, than lime and limestone and are more often used:    1. where the volume of waste gas to be treated is small (compared to those from large power plants);    2. where other factors such as transportation cost of the alkali material is economical;    3. where required or necessitated by local or regional regulatory constraints; or    4. where any combination of these and other economic, technical, or regulatory issues make this alternative economically and environmentally viable.
Some of these alternative alkali materials used in flue gas treatment are dry sodium-based sorbents which include sodium carbonate (Na2CO3), sodium bicarbonate (NaHCO3), sodium sesquicarbonate (Na2CO3.NaHCO3.2H2O), combinations thereof, or minerals containing them such as trona, nahcolite.
Trona, sometimes referred to as sodium sesquicarbonate (Na2CO3.NaHCO3.2H2O) due to its high content in sodium sesquicarbonate (typically 70-99 wt %), is a natural mineral and is receiving increased widespread use in dry flue gas treatment systems. Nahcolite, sometimes referred to as sodium bicarbonate (NaHCO3), is also a natural mineral which may be used in dry or slurry flue gas treatment systems.
For dry sorbent injection, dry powdered sodium-containing sorbent (such as particulate trona or sodium bicarbonate) is injected into an air duct through which a flue gas stream (containing combustion solid matter and gaseous acidic combustion byproducts) flows. The acidic gases and the sodium-containing sorbent (e.g., trona or sodium bicarbonate) react to form treatment byproducts. The solid components of the treated flue gas including combustion solid matter, treatment by-products (which may be solid sodium salts and/or may be adsorbed/absorbed on the combustion solid matter), and optionally any unreacted sodium-containing sorbent (when a stoichiometric excess is used) are removed from the flue gas stream using a particulate recovery system such as one or more baghouse filters or preferably one or more electrostatic precipitators (ESP) to collect solids referred to as a ‘sodic fly ash’ and to recover a DSI-treated flue gas stream which may be further subjected to a wet scrubber to further remove remaining acid gaseous combustion byproducts.
One example of a flue gas desulfurization treatment using a sodium-based dry sorbent injection technology is described in U.S. Pat. No. 7,854,911 by Maziuk. Maziuk describes the chemical reaction of trona with SO2, which unlike sodium bicarbonate, melts at elevated temperatures. According to Maziuk, trona (mainly sodium sesquicarbonate) undergoes rapid calcination of contained sodium bicarbonate to sodium carbonate when heated at or above 275° F. Maziuk suggests that the “popcorn like” decomposition creates a large and reactive surface by bringing unreacted sodium carbonate to the particle surface for SO2 neutralization. The byproduct of the reaction is sodium sulfate and is collected in the fly ash. The chemical reaction of the trona with the SO2 is represented below:2[Na2CO3.NaHCO3.2H2O]→3Na2CO3+5H2O+CO2 Na2CO3+SO2→Na2SO3+CO2 Na2SO3+1/2O2→Na2SO4 
Other reactions with trona when injected into flue gas of a coal-fired power plant, may include a reaction with hydrochloric acid according to the following:[Na2CO3.NaHCO3.2H2O]+3HCl→3NaCl+4H2O+2CO2 
The solid reaction products of the trona and the acid gases (e.g., SO2, SO3, HF, HCl) which are primarily sodium salts (e.g., sodium sulfate, sodium sulfite, sodium fluoride, and/or sodium chloride) as well as unreacted sodium carbonate are then collected in one or more particulate collection devices, such as baghouse filter(s) or electrostatic precipitator(s).
For example, trona may be maintained in contact with the flue gas for a time sufficient to react a portion of the trona with a portion of the SO3 to reduce the concentration of the SO3 in the flue gas stream. For SO3 removal, the total desulfurization is preferably at least about 70%, more preferably at least about 80%, and most preferably at least about 90%.
Whenever possible, fly ash resulting from the combustion of coal (‘coal fly ash’) which is collected from the particulate recovery system may be used in various applications; otherwise dry fly ash is disposed into a landfill. Typical coal fly ash is mainly composed by aluminosilicates partially vitrified, as well as mineral phases such as quartz, hematite, maghemite, anhydrite and so on which had been present as impurities in the original coal. Sodic fly ash further comprises spent sodium-based sorbent.
ASTM C 618-85 (“Standard specification for fly ash and raw calcinated natural pozzolan for use as a mineral admixture in Portland cement concrete”) has classified fly ash into two classes, Class C and Class F, depending on the total sum of silica, alumina and ferric oxide present. Class F contains more than 70 percent of the above oxides and Class C contains less than 70 percent but more than 50 percent. Class F fly ash is typically low in calcium oxide (<8 percent) whereas Class C has a higher content being sub-classified in two categories: Class Cl (8-20 percent CaO) and Class CH (>20 percent CaO). Therefore, Class F fly ash is not usually considered as a cementitious material by itself because, due to its low calcium oxide content, it cannot be agglomerated after hydration to produce bonding strength in the final product, contrary to Class C fly ash.
Fly ash is a by-product that has to be used and consumed to reduce its environmental impact. Nowadays, it has mainly been used as a partial substitute in ordinary Portland cement due to its pozzolanic reactivity. However, there is a limitation in the replaced quantity because the pozzolanic reaction rate is very low at room temperature causing initial low strength and fast neutralization.
In 2006, U.S. coal-fired power plants have generated 72 million tons of fly ashes. Almost 45% of these solid residues (32 million tons) are used in a dozen of applications. According to AMERICAN COAL ASH ASSOCIATION, “2006 Coal Combustion Product (CCP)—Production and Use Survey”, among these applications,                15 million tons of fly ashes are used in concrete/concrete products/grout;        7 million tons of fly ashes are used in structural fills/embankments; and        4 million tons of fly ashes are used in cement/raw feed for clinker.        
Sodic fly ashes resulting from flue gas acid gas removal treatment which predominately use powdered trona or sodium bicarbonate as sodium-based sorbent in DSI systems contain not only fly ash particles coated and intermixed with water-soluble sodium salts (e.g., sodium sulfite, sulfate, chloride, and/or fluoride) and unreacted sodium-based sorbent, but also contain various metallic compounds and other chemical attributes that may pose an environmental concern if the sodic fly ashes are placed in a landfill or used for beneficial re-use.
Even though trona or sodium bicarbonate use for acid gas removal from flue gases of coal-fired power plants has been helpful to address regulatory constraints in the United States, these sodium-based sorbents have modified the physical and chemical characteristics of the fly ashes with two consequences which are as follows:                the leaching of trace elements (such as Se, As, Mo) and soluble matter increases with sodium content and alkalinity: it raises the question of its impact on the environment (environmental storage management, surface and ground water quality, human health . . . ), and        the high content of water-soluble sodium salts may certainly prevent from the possible valorization of the sodic fly ashes into concrete if done without any further treatment (Standard ASTM-C-618: as a pozzolanic additive, fly ash must not content more than 1.5 wt % of Na2O) and also raises the issue of its storage.        
Resulting from the introduction of the sodium-based sorbent, some water-soluble sodium-heavy metal complexes, compounds, and the like, may be formed, when heavy metals contained in the flue gas get in contact with the sodium-based sorbent. As the formation of water-soluble matter with fly ash trace elements (such as Se) increases with sodium content, so does the leachability of some of these trace elements from the sodic fly ash.
In an Electric Power Research Institute Report No. 1017577 (2010) entitled “Impacts of Sodium-based Reagents on Coal Combustion Product Characteristics and Performance”, it was reported that greater than 50% of the sodium leached in all leachates from the sodium-based reagent coal combustion product samples (CCP) while less than 15% of the sodium leached from standard CCP samples. This indicates that the added sodium was more mobile than the inherent sodium from the coal in the standard CCPs. It was also remarked that selenium and arsenic were generally more mobile in the leachates from CCP samples with sodium-based sorbent injection than in the standard CCP samples. It was noted that the highest vanadium leachate concentrations in the sample set were from the CCP sample with sodium carbonate injection.
Jianmin Wang and coworkers also studied the impact of trona injection on the characteristic of the resulting fly ash and on the leaching characteristics of anionic elements, including As, Se, Mo, and V.
In Su et al., “Impact of Trona-Based SO2 Control on the Elemental Leaching Behavior of Fly Ash” Energy Fuels, 2011, Vol. 25, pg. 3514-3521, and in Dan et al, “Increased Leaching of As, Se, Mo, and V from High Calcium Coal Ash Containing Trona Reaction Products” Energy Fuels, 2013, vol. 27, pp 1531-1537, it was shown that trona injection and subsequent capture of the reaction products with fly ash significantly enhanced the leaching of As, Se, Mo, and V. Their results also indicated that, with trona addition, the distribution of these anions shifted to the soluble trona fraction of the ash. Therefore, the dissolution of the spent trona sorbent resulted in more leaching of these anionic elements. In addition, they found that trona injection significantly reduced the adsorption capability of the insoluble fraction of the ash for As, Se, and V under the natural pH, and made them more leachable. For use in cement and concrete applications, a number of strategies have been developed over the last 50 or more years for effectively designing concrete with pozzolans such as coal fly ash. A pozzolan is broadly defined as an amorphous or glassy silicate or aluminosilicate material that reacts with calcium hydroxide formed during the hydration of Portland cement in concrete to create additional cementitious material in the form of calcium silicate and calcium silicoaluminate hydrates. However it has been established that pozzolans must be low in alkalis (Na2O and K2O), to avoid long-term durability problems in concrete by expansion due to alkali-silica reactions.
If the valorization (such as use in cement and concrete) or landfilling of a sodic fly ash may be problematic due to high sodium content and leachability of some heavy metals result in exceeding the maximum allowed content limits in leachates set by local, state and/or federal regulations for leaching, the sodic fly ash may need to be processed to satisfy these requirements for valorization or landfill.
At an industrial scale, a wet treatment of sodic fly ash would include solubilization of water-soluble components from the sodic fly ash (which are mostly spent sorbent with unreacted sorbent and pollutants' reaction by-products), a liquid/solid separation and a subsequent treatment of leachates with high levels of Na, sulfate, carbonate, hydroxide, and some heavy metals (particularly selenium, arsenic and molybdenum). But this approach displaces the fly ash disposal issue to a wastewater management issue.
The primary leachate concerns are selenium and arsenic. The proposed Effluent Limitation Guidelines (ELG) propose leachate water limits for arsenic of 8 μg/L single day and 6 μg/L on a 30-day rolling average. For selenium, the leachate water limits are 16 μg/L for a single day and 10 μg/L on a 30-day rolling average.
In particular, if the leachate in an untreated trona-based fly ash provided by coal combustion may generate a leachate with a content in selenium (Se) or arsenic (As) above the regulatory limits, such sodic coal fly ash must be treated prior to land disposal or beneficial re-use.
The Resources Conservation and Recovery Act (RCRA) of 1976 is the principal federal law in the United States governing the disposal of solid waste and hazardous waste. The maximum acceptable leachate concentration for selenium into a RCRA Subtitle D landfill is one (1) mg/L; and the maximum acceptable leachate concentration for arsenic into a RCRA Subtitle D landfill is five (5) mg/L. Fly ashes that exceed these limits would be classified as hazardous wastes and be more expensive to landfill. In these cases it would be cost effective to treat the fly ash to avoid the hazardous classification and reduce disposal costs.
Selenium in particular is a difficult metal to treat because selenium (Se) exhibits a variety of oxidation states. In an alkaline environment under slightly oxidizing conditions, the selenate (Se+4, SeO4−2) ion predominates. Conversely, in an acidic environment that is still oxidizing, the selenite (Se+3, SeO3) ion predominates. Selenate is significantly mobile in soils with little adsorption of the selenate ion over a pH range of 5.5-9.0. Therefore, selenium mobility is favored in oxidizing environments under alkaline conditions. As a result, the concentration and form of selenium is governed by pH, redox, and matrix composition (e.g., soil, ash) and makes short term and long term treatment difficult in various environments, but particularly difficult for sodic fly ash at elevated pH when excess sodium-based sorbent such as trona (Na2CO3.NaHCO3.2H2O) is used in flue gas treatment. Reported pH for sodic fly ashes has been from about 10.5 to about 12.8.
Water-soluble heavy metal compounds (such as selenate and/or selenite) may be detrimental if they leach from the fly ash. Sodium salts leaching from a landfill usually are not hazardous, but the leaching of soluble materials from a landfill can impact the structural integrity of the pile and how the landfill is managed. Proctor tests provide some insight into density and moisture properties but do not measure how rain and other factors affect the physical characteristics of the landfill. Thus there is a need for more investigation and obtaining data from actual landfills.
Hence here lies a dilemma for the power plant operators. On one side, one needs to reduce the amounts of gaseous pollutants emitted by combustion processes (such as coal-fired power plants), while due to the nature of the fuel necessitating chemical treatments for pollutant control, there is an increased generation of combustion wastes containing heavy metals such as Se and As and resulting in an increase need in disposal or valorization of solid wastes obtained therefrom.
Additionally, if in order to address the increased leachability of some heavy metals (mostly oxyanions) from sodic fly ash, the wet processing approach is likely avoided since it results in dissolving the water-soluble components of fly ash (mostly spent sodium-based sorbent, reaction byproducts, and leachable heavy metals) and then in treating the resulting wastewater. One might have to envision a dry processing approach for stabilization of sodic fly ash. However, the handling of such dry material poses additional concern relating to fugitive dust. Dust control thus may need to be addressed and may become an integral part of such a stabilization method.